1. Field of the Invention
The present invention relates to a high speed reluctance motor, and more particularly to a start assisting device for a high speed reluctance motor, capable of starting the motor at a specific start disabling position where a static torque with a very small angle is present due to a phase advance.
2. Description of the Prior Art
FIG. 1 is a schematic sectional view of a conventional three-phase reluctance motor with a stator and a rotor. As shown in FIG. 1, the reluctance motor comprises a stator 3 including six fixed magnetic poles 3a mounted to a motor housing and spaced uniformly from one another at 60.degree. and coils 4 wound around one of pairs of opposed fixed magnetic poles 3a to form magnetic poles with reversed polarity. With this arrangement, the pairs of opposed magnetic poles have different phases a, b and c, respectively. The reluctance motor also comprises a rotor 2 disposed inwardly of the stator 3 to be rotatable about a rotating shaft 1 and provided with four rotating magnetic poles 4a space uniformly from one another at 90.degree..
In such a conventional reluctance motor, a rotor position detecting device (not shown) is also provided to detect the position of rotor 2. Based on the detected position of rotor 2, electric power is sequentially applied to coils 4 wound around the fixed poles 3a and provided with phases a, b and c, to drive the motor.
In the reluctance motor, a phase advance of .alpha. occurs at an interval .beta. where an optional phase is turned on during the rotation of reluctance motor at a high rate, that is, a magnetic force is generated from magnetic poles 3a with a required phase, due to a time taken until current applied to coils 4 reaches a normal state, as apparent from a static torque curve shown in FIG. 2.
The angle .alpha. is larger at a higher inductance of the coils and a higher speed of the motor. When the angle .alpha. exceeds a certain level, the interval .beta. involves a point of time at which the static torque is zero, that is, a point A in FIG. 2.
The point A of FIG. 2 at which the static torque is zero is present in the form of an interval other than a point form, as shown in FIG. 3, due to friction and load conditions of the motor.
There is few problem in starting the motor, when the total interval (.alpha.+.beta.) does not exceed the point A. However, when the total interval (.alpha.+.beta.) exceeds the point A, the motor may be stopped at a position where a torque generated by driving an optional phase, that is, at the point A, so that the motor can not be started.
For example, if a phase to be driven for starting the motor is the phase a-a' under a condition that the rotor 2 has been stopped such that the rotating magnetic poles 4a of rotor 2 has been symmetrically positioned with respect to fixed magnetic poles with the phase a-a', as shown in FIG. 1, a rotation force can not be generated since the rotating magnetic poles of rotors 2 and the fixed magnetic poles are positioned symmetrically in all directions, even though the phase a-a' is driven. As a result, an initial start of the motor can not be accomplished.
On the other hand, where conduction angles Z2 and Z4 are smaller than a step angle Z1, for the purpose of improving the efficiency of motor, as shown in FIG. 4, any phase can not be driven at positions corresponding the angles Z3 and Z5. When the motor is stopped at such positions, its start can not be accomplished. That is, any drive signal for initially driving the motor can not be generated when the motor has been stopped at the positions corresponding to the angles Z3 and Z5, since drive signals for the phases of motor (the phases a, b and c, in the illustrated three-phase motor) are intermittently generated with certain intervals corresponding to the angles Z3 and Z5.
In the conventional reluctance motor, a drive signal for each phase is generated only by a detecting signal for the rotor. Accordingly, there is a problem that the start of motor can not be accomplished at the above-mentioned specific positions, when the phase advance interval is increased due to a high speed of motor or a high inductance of coils 4 or when the conduction angle is smaller than the step angle for the purpose of improving the efficiency of motor. As a result, the design of motors is restricted by the phase advance angle and conduction angle, thereby causing the speed and efficiency of high speed reluctance motors to be degraded.